1. Field of the Invention
The present invention relates to an optical head apparatus and in particular, to an optical head apparatus causing no offset in a track error signal even if an objective lens is shifted and capable of detecting a land/groove position.
2. Description of the Related Art
In a conventional optical head apparatus, the push-pull method is known as one of the track error signal detecting methods. The push-pull method is realized by a simple configuration of an optical system and an electric circuit but an offset is caused in the error detecting signal if an objective lens is shifted.
To cope with this, there is known a method to use a diffraction grating to generate three beams of 0-th order light, plus and minus 1.sup.st -order diffracted lights, so that the offset of the track error signal caused by the objective lens shift is cancelled by a difference between the 0-th order light and the plus and minus 1.sup.st -order diffracted lights. An optical recording medium has a land and a groove. In this method, the 0-th order light is applied to the land (or the groove) and the plus and minus 1.sup.st -order diffracted lights are applied to the adjacent grooves (or lands). However, in an optical recording medium having a track pitch different from a design, it is impossible to apply the three focal spots as mentioned above and accordingly, it is impossible to cancel the track error signal offset upon an objective lens shift.
Japanese Patent Publication (Unexamined) No. 9-81942 discloses a method to use a diffraction grating consisting of two regions one of which has a phase delayed by .pi. from the phase of the other, so as to generate three beams of 0-th order light and plus and minus 1.sup.st -order diffracted lights so that differences between the 0-th order light and the plus and minus 1.sup.st -order lights are used to cancel a track error signal offset at an objective lens shift. In this method, the 0-th order light and the plus and minus 1.sup.st -order diffracted lights is applied to a single land (or groove). Consequently, even in an optical recording medium having a track pitch different from a predetermined design, the arrangement of the three focal spots is not changed, enabling to cancel the offset of the track error signal caused by the objective lens shift.
FIG. 17 shows a configuration of a conventional optical head apparatus using the aforementioned method.
A light emitted from a semiconductor laser 51 is made into parallel lights by a collimator lens 52 and divided by a diffraction grating 53d into 0-th order light and plus and minus diffracted lights. Approximately half of these lights are passed through a beam splitter 54 and focused by an objective lens 55 on a disc 56. The three lights reflected from the disc 56 are introduced via the objective lens 55 into the beam splitter 54, where about half of the lights is reflected to be received via a composite lens 57 by a photo detector 58d. The composite lens 57 consists of a convex lens and a cylindrical lens. The photo detector 58d is arranged in an intermediate position between two focal lines of the composite lens 57.
FIG. 18 is a plan view of the diffraction grating 53d. The diffraction grating 53d is divided into a region 78a and a region 78b. The line of this division is a straight line in a tangential direction (parallel to the track) passing through the optical axis of the incident light 59. The phase difference between the region 78a and the region 78b is .pi.. Accordingly, there is a phase difference of .pi. between the plus and minus 1.sup.st -order diffracted lights from the region 78a and the plus and minus 1.sup.st -order diffracted lights from the region 78b.
FIG. 19 shows an arrangement of the focal spots on the disc 56. The 0-th order light, the plus 1.sup.st -order diffracted light, and the minus 1.sup.st -order diffracted light respectively correspond to focal spots 79a, 79b, and 79c, which are arranged on a single track 61 (land or groove). The focal spots 79b and 79c have two peaks having an identical intensity in a radial direction (vertical direction to the track).
FIG. 20 shows light receiving blocks of the photo detector 58d and a light spot arrangement on the photo detector 58d. A light spot 80a corresponds to the 0-th order light which is received by the light receiving block divided into four light receiving sections 81a to 81d by two straight lines of tangential direction passing through the optical axis and the radial direction. A light spot 80b corresponds to the plus 1st-order diffracted light, which is received by a light receiving block divided into a light receiving sections 81e and 81f by a tangential line passing through the optical axis. A light spot 80c corresponds to the minus 1.sup.st -order diffracted light, which is received by a light receiving block divided into light receiving sections 81g and 81h by a tangential line passing through the optical axis. The focal spots 79a, 79b, and 79c are arranged in the tangential direction on the disc 56, but the light spots 80a, 80b, and 80c on the photo detector 58d are arranged in the radial direction by the function of the composite lens 57.
If it is assumed that outputs of the light receiving sections 81a to 81h are V81a to V81h, the focus error signal can be obtained from the calculation (V81a+V81d)-(V81b+V81c) according to the astigmatism. The track error signal can be obtained by the differential push-pull method as follows: {(V81a+V81b)-(V81c+V81d)}-K{(81e+V81g)-(V81f+V81h)} (wherein K is a constant). Moreover, the reproduction signal can be obtained from the calculation of V81a+V81b+V81c+V81d.
FIG. 21 shows a phase change of the 0-th order light, the plus and minus 1.sup.st -order diffracted lights from the disc 56 caused by a position difference between the focal spot 79a on the disc 56 and the track 61. The focal spot 79a is formed by a beam 66d.
FIG. 21A, case (1), the light beam 66d is applied to a groove 67a. Here, if the 0-th order light is assumed to have phase 0, the plus and minus 1.sup.st -order diffracted lights have a phase of -.pi./2. In FIG. 21A, case (2), the light beam 66d is applied to a boundary of the groove 67a and the land 67b. Here, with respect to case (1), the plus 1.sup.st -order diffracted light has a phase delayed by .pi./2, and the minus 1.sup.st -order diffracted light has a phase advancing by .pi./2. Accordingly, if the 0-th order light has a phase 0, the plus 1.sup.st -order diffracted light has a phase of plus and minus .pi., and a minus 1.sup.st -order diffracted light has a phase 0. In FIG. 21A, case (3), the light beam 66d is applied to the land 67b. Here, with respect to case (2), the plus 1.sup.st -order diffracted light has a phase delayed by .pi./2, and the minus 1.sup.st -order diffracted light has a phase advancing by d .pi./2. Accordingly, if the 0-th order light has phase 0, the plus and minus 1.sup.st -order diffracted lights have a phase of .pi./2. In FIG. 21A, case (4), the light beam 66d is applied to a boundary between the land 67b and the groove 67a. Here, with respect to case (3), the plus 1.sup.st -order diffracted light has a phase delayed by .pi./2, and the minus 1.sup.st -order diffracted light has a phase advancing by .pi./2. Accordingly, if the 0-th order light has phase 0, the plus 1.sup.st -order diffracted light has phase 0 and the minus 1.sup.st -order diffracted light has phase plus and minus .pi..
FIG. 21B shows a region 82a containing both of the 0-th order light and the plus 1.sup.st -order diffracted light and a region 82b containing both of the 0-th order light and the minus 1.sup.st -order diffracted light. These regions 82a and 82b have light intensities as follows. In FIG. 21A, case (1), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light and the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light are both .pi./2. Accordingly, the light intensity of region 82a is equal to the light intensity of region 82b. In FIG. 21A, case (2), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light is .pi., and their intensities are weakened by interference. On the other hand, the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light is 0 and their intensities are intensified by interference. Accordingly, the intensity of the region 82a is low and the intensity of the region 82b is high. In FIG. 21A, case (3), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light and the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light are both .pi./2. Accordingly, the light intensity of region 82a is equal to the light intensity of region 82b. In FIG. 21A, case (4), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light is 0 and their intensities are increased by interference. On the other hand, the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light is .pi. and their intensities are weakened by interference. Accordingly, the intensity of the region 82a is high and the intensity of the region 82b is low.
FIG. 22 shows a focal spot 79b and phase changes of the 0-th order light and the plus and minus 1.sup.st -order diffracted light from the disc 56 caused by a position shift of the track 61. The focal spot 79b is formed by a light beam 66e. The light beam 66e has a phase at the right side shifted by .pi. from a phase at the left side.
In FIG. 22A, case (1), the light beam 66e is applied to the groove 67a. Here, if it is assumed that the 0-th order light has a phase -.pi./2 at the left half and .pi./2 at the right half, the plus and minus 1.sup.st -order diffracted light has a phase -.pi. at the left side and phase 0 at the right side. In FIG. 22A, case (2), the light beam 66e is applied to a boundary between the groove 67a and the land 67b. Here, with respect to case (1), the plus 1.sup.st -order diffracted light has a phase delayed by .pi./2 and the minus 1.sup.st -order diffracted light has a phase advanced by .pi./2. Accordingly, if it is assumed that the 0-th order light has a phase -.pi./2 at the left side and a phase .pi./2 at the right side, then the plus 1.sup.st -order diffracted light has a phase .pi./2 at the left side and a phase -.pi./2 at the right side, and the minus 1.sup.st -order diffracted light has a phase -.pi./2 at the left side and a phase .pi./2 at the right side. In FIG. 22A, case (3), the light beam 66e is applied to the land 67b. Here, with respect to case (2), the plus 1.sup.st -order diffracted light has a phase delayed by .pi./2 and the minus 1.sup.st -order diffracted light has a phase advanced by .pi./2. Accordingly, if it is assumed that the 0-th order light has a phase -.pi./2 at the left side and a phase .pi./2 at the right side, then each of the plus and minus 1.sup.st -order diffracted light has a phase 0 at the left side and a phase .pi. at the right side. In FIG. 22A, case (4), the light beam 66e is applied to the boundary between the land 67b and the groove 67a. Here, with respect to case (3), the plus 1.sup.st -order diffracted light has a phase delayed by .pi./2 and the minus 1.sup.st -order diffracted light has a phase advanced by .pi./2. Accordingly, if it is assumed that the 0-th order light has a phase -.pi./2 at the left side and a phase .pi./2 at the right side, then the plus 1.sup.st -order diffracted light has a phase -.pi./2 at the left side and a phase .pi./2 at the right side, and the minus 1.sup.st -order diffracted light has a phase .pi./2 at the left side and a phase -.pi./2 at the right side.
FIG. 22b shows a region 83a containing both of the 0-th order light and the plus 1.sup.st -order diffracted light and a region 83b containing both of the 0-th order light and the minus 1.sup.st -order diffracted light. These regions have light intensities as follows. In FIG. 22A, case (1), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light and the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light are both .pi./2. Accordingly, the light intensity of region 83a is equal to the light intensity of region 83b. In FIG. 22A, case (2), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light is 0, and their intensities are increased by interference. On the other hand, the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light is .pi. and their intensities are weakened by interference. Accordingly, the intensity of the region 83a is high and the intensity of the region 83b is low. In FIG. 22A, case (3), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light and the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light are both .pi./2. Accordingly, the light intensity of region 83a is equal to the light intensity of region 83b. In FIG. 22A, case (4), the phase difference between the 0-th order light and the plus 1.sup.st -order diffracted light is .pi. and their intensities are weakened by interference. On the other hand, the phase difference between the 0-th order light and the minus 1.sup.st -order diffracted light is 0 and their intensities are increased by interference. Accordingly, the intensity of the region 83a is low and the intensity of the region 83b is high.
FIG. 23 shows various waveforms related to the track error signal. The horizontal axis represents a positional difference between the focal spot on the disc 56 and the track 61. Arrows a to d respectively correspond to the cases (1) to (4) in FIG. 21A and FIG. 22A.
The region 82a in FIG. 21B corresponds to the light receiving sections 81c and 81d of the photo detector 58d. The region 82b in FIG. 21B corresponds to the light receiving sections 81a and 81b of the photo detector 58d. Here, the waveform of (V81a+V81b)-(V81c+V81d) is shown by a solid line in FIG. 23A. The region 83a in FIG. 22B corresponds to the light receiving section 81f of the photo detector 58d. The region 83b in FIG. 22B corresponds to the light receiving sections 81e of the photo detector 58d. Here, the waveform of (V81e-V81f) is shown by a solid line in FIG. 23B. Similarly, the waveform of (V81g-V81h) is as shown by a solid line in FIG. 23C. From the waveforms of FIG. 23B and FIG. 23C, the waveform of (V81e+V81g)-(V81f+V81h) is as shown by a solid line in FIG. 23D. Because of the waveforms of FIG. 23A and FIG. 23D having phases reversed to each other, the waveform of {(V81a+V81b)-(V81c+V81d)}-K{(V81e+V81g)-(V81f+V81h)} is as shown by a solid line in FIG. 23E.
When the objective lens is shifted in the radial direction, the light spots 80a to 80c on the photo detector 58d are also shifted in the radial direction. If it is assumed that the light spots 80a to 80c are shifted upward in FIG. 20, the outputs of the light receiving sections 81a and 81b are increased and the outputs of the light receiving sections 81c and 81d are decreased. Accordingly, the waveform of (V81a+V81b)-(V81c+V81d) is as shown by a dotted line in FIG. 23A. Moreover, the output of the light receiving section 81e is increased and the output of the light receiving section 81f is decreased. Accordingly, the waveform of (V81g-V81h) is as shown by a dotted line in FIG. 23B. Similarly, the waveform of (V81g-V81h) is as shown by a dotted line in FIG. 23C. From the dotted lines in FIG. 23B and FIG. 23C, the waveform of (V81e+V81g)-(V81f+V81h) becomes as shown by a dotted line in FIG. 23D. The waveforms of FIG. 23A and FIG. 23D have phases reversed to each other but DC components at the objective lens shift have identical signs. Accordingly, the track error signal {(V81a+V81b)-(VB1c+V81d)}-K{(V81e+V81g)-(V81f+V81h)} has a waveform as shown by a solid line in FIG. 23E. That is, even if the objective lens is shifted, no offset is caused in the track error signal.
Here, in the optical head apparatus, when accessing the land (or the groove), in order to prevent run-away of the track servo, it is preferable to pull in the track servo after confirming that the focal spot is on the land (or the groove). For this, it is necessary to provide a land/groove position detecting function for detecting on which of the land and groove the focal spot resides.
However, in a conventional optical head apparatus, there is a problem that it is not always possible to detect the land/groove position. In case the groove 67a and the land 67b have different widths, the level of (V81a+V81b+V81c+V81d) varies depending on whether the focal spot 79a is on the groove 67a or on the land 67b. This enables to detect the land/groove position. However, in case when the groove 67a and the land 67b have identical widths, the level of (V81a+V81b+V81c+V81d) is identical when the focal spot 79a is on the groove 67a and when on the land 67b. Accordingly, it is impossible to detect the land/groove position.